In contact coagulation of biological tissue, primarily thermally induced processes take place that lead to, among other things, to the denaturing of the tissue whereby any hollow vessels that are present are intended to be closed. When applying a high-frequency voltage to biological tissue a very high electric impedance of the tissue can be observed at the beginning of the process. The current of electrical charge carriers passes primarily through extracellular fluids as a result of which the tissue starts to heat up due to the kinetic energy of the displaced carriers of the electric charge. As the tissue is increasingly heated, the impedance decreases until it reaches a minimum. The increase of the electric conductivity occurs in a temperature range of 60° C. to 100° C. of the biological tissue due to its temperature-induced structural changes that accompany the denaturing of the tissue. The tissue is devitalized; the protein molecule clump together, the cell membrane is destroyed as a result of which tissue fluid is released. In this “phase I”, the tissue impedance Z continually decreases. After some time, the boiling temperature of the tissue fluid is reached, whereby the tissue resistance once again increases, which is described as “phase II”. Generally, the tissue impedance Z reaches values in phase II that are clearly above the impedance minimum of the tissue and often the initial impedance in phase I.
EP 2 520 240 A1 discloses a method and an arrangement for tissue fusion and also for coagulation in which by specifying a negative internal resistance of a supplying high-frequency source, a constantly consistent treatment time is intended to be achieved.
EP 1 862 137 A1 discloses a device and a method for coagulation of tissue in which the tissue impedance Z is polled and monitored. By means of continual readjustment of the electrical energy delivered to the tissue it is achieved that the impedance of the tissue follows a desired, specified curve. In particular, this applies to phase II.
Further, DE 36 22 337 A1 discloses a high-frequency generator with automatic power control for a high-frequency coagulation that has an electric arc display device to detect an arc between the coagulation sensor and the tissue. In order to securely spark the electric arc, initially, the maximum amount of output power is used. After the electric arc is sparked, at first maximum power continues to be supplied for a certain period of time. The output power is then reduced to zero for a predetermined second interval. As long as the generator is activated, these cycles continue to be repeated.
By means of the method described by DE 36 22 337 A1, a coagulation mode is achieved in which the coagulation
initially starts with contact coagulation, whereby after reaching the boiling temperature of the tissue fluid, an electric arc penetrates the vapor that is forming, whereby the current density is highly elevated at the penetration site of the electric arc, as a result of which a marked local coagulation effect occurs and the tissue takes on high impedance. The electric arc sparks and jumps to various locations until the entire tissue in the proximity of the coagulation sensor has high impedance, i.e. has coagulated. Switching off of the electric arc intermittently by setting the output line to zero prevents excessive burning, i.e. an overly strong carbonization of the tissue.
In the case of a fast coagulation that is based on the formation of sparks or which permits such, the tissue can adhere to the instrument and thus lead to an accompanying significant contamination of the instrument and also the treatment personnel. Moreover, the carbonization can impede the wound-healing process.
If a fast coagulation without an electric arc is brought about by pure contact coagulation in phase I with elevated high-frequency power delivered by a high-frequency generator, an acutely audible and visually perceivable tearing of the treated tissue can occur. This is caused by local tearing of tissue due to boiling tissue fluid and the accompanying increase in tissue pressure. Due to the tearing of the tissue, previously stopped bleeding can start to bleed anew. Furthermore, treated pathogenic tissue can disseminate into healthy tissue areas due to the tearing and also be absorbed by the attending personnel.